The invention relates to a process for suitably purifying 2-pyrrolidones for subsequent polymerization to polypyrrolidone having a sufficiently high molecular weight.
Suitable methods for preparing 2-pyrrlidones by hydrogenating the corresponding succinonitriles followed by reacting water with the hydrogenation product are known, see for instance, U.S. Pat. Nos. 3,644,402; 4,123,438; and 4,193,925. According to these methods an aqueous pyrrolidone-containing solution is obtained from which fairly pure pyrrolidones can be recovered, for example, by fractional distillation. However, these recovered pyrrolidones do not satisfactorily polymerize to polypyrrolidones.
The many already proposed solutions for purifying the pyrrolidones have several drawbacks. Such drawbacks include, unacceptably high pyrrolidone losses, compatibility only with pyrrolidones produced from other than succinonitrile, or pyrrolidone products having a degree of purity unacceptably lower than that required for polymerizing pyrrolidones to polypyrrolidones.
Pyrrolidone can be purified in the liquid phase by dissolving therein a substantially water free solid hydroxide, such as KOH, and then subjecting the solution to a flash distillation using a molecular vacuum distillation apparatus having a heated flash vaporization surface, as described in U.S. Pat. No. 4,014,900. However, while that method achieves a pyrrolidone of a good degree of purity, the impurities and pyrrolidone losses amount to about 5% to 10% by weight of the starting material.
The process of the present invention provides a method for purifying 2-pyrrolidones obtained from the corresponding succinonitriles to yield a 2-pyrrolidone product of polymerization grade purity while minimizing the unrecoverable pyrrolidone losses.